What is the minimum allowable discharge voltage for maximum life of a standard 12-volt lead-acid battery?

A standard 12-volt lead-acid battery is made up of six cells, each providing about 2.1 volts, which results in a fully charged resting voltage of approximately 12.6 volts. To maintain the maximum lifespan of the battery and prevent permanent chemical damage, it should never be completely drained. The generally accepted absolute minimum discharge voltage under load is 10.5 volts, which equates to 1.75 volts per cell. Draining the battery below this 10.5-volt threshold causes severe sulfation on the lead plates, permanently reducing the battery's capacity and overall life.